FIELD OF THE INVENTION
The invention relates to a method for the cylinder-selective knock control of an internal combustion engine, in which a predetermined cylinder-selective basic ignition angle associated with knock-free operation and a cylinder-selective knock adjustment angle that increases stepwise with a predetermined step size in the retarded direction each time a knock occurs and decreases stepwise with a predetermined step size in the advanced direction after each engine cycle during knock-free operation form a cylinder-selective ignition angle in association with the respective operating point, which is dependent on load and engine speed. A method of that kind is known from German published patent application DE 29 25 770 A1.
When engine knock occurs in a cylinder z of the internal combustion engine, the ignition angle for this cylinder is retarded by a certain amount--step size SK.sub.dec --thereby reducing the probability that knocking combustion will occur in this cylinder. If the engine then operates without knock, the ignition angle is slowly advanced again by a predetermined amount SK.sub.inc.
The known total ignition angle ZW(z) for a cylinder z at a particular operating point is made up of a basic ignition angle GZ(z) for knock-free operation, which is dependent on the load L and the engine speed n, is stored in a map and--in the case of a four-cylinder engine--is updated every 180.degree. of crank angle (hence the term "cylinder-selective"), this basic angle being calculated from the ignition dead center position ZOT(z) closest to ignition, and of an additional knock adjustment angle KNK(z) for this cylinder z owing to engine knock: ZW(z)=GZ(z)-KNK(z). It should be noted here that the knock adjustment angle KNK(z) can assume only negative values, in line with the recognition that a positive sign signifies "advance", while a negative sign signifies "retardation".
The respective knock adjustment angles KNK(z) are entered in one load- and engine-speed-dependent map per cylinder. As the change from one operating point to the next occurs, the last knock adjustment angle KNK(z) entered is stored at the old operating point. When the engine re-enters this operating point, this value is reused as the knock adjustment angle KNK(z) for knock control. This method has the disadvantage that a relatively random knock adjustment angle will be stored, depending on the time at which the change in the operating point occurs.
This results in the following disadvantages:
the knock limit is not adapted accurately; PA1 the ignition-angle profile in the case of transitions between the adaptation ranges is nonuniform, and the torque profile is therefore not continuous either; PA1 the internal combustion engine is not operated exactly at the knock limit; as a result, optimum torque and optimum specific fuel consumption are not assured. PA1 determining a cylinder-selective first adaptation value of a first adaptation circuit from a comparison of the knock adjustment angle with a first and a second threshold value; PA1 wherein the first threshold value is greater than the step size, specified in the retarded direction, of the cylinder-selective knock adjustment angle, and the second threshold value is less than the step size, specified in the retarded direction, of the cylinder-selective knock adjustment angle; PA1 modifying the cylinder-selective adaptation value of the first adaptation circuit in the retarded direction with a predetermined first adaptation step size after each engine cycle for as long as the knock adjustment angle is greater in terms of an absolute value thereof than the first threshold value; PA1 holding the cylinder-selective adaptation value of the first adaptation circuit constant for as long as the knock adjustment angle is less than the first threshold value and greater than the second threshold value in terms of the absolute value; and PA1 modifying the cylinder-selective adaptation value of the first adaptation circuit in the advanced direction with a predetermined second adaptation step size for as long as the knock adjustment angle is less in terms of the absolute value than the second threshold value; PA1 determining a second adaptation value, associated with all the cylinders, of a second adaptation circuit from a comparison of the average value of all the cylinder-selective adaptation values of the current operating point with a predetermined threshold; and PA1 using the cylinder-selective first adaptation value and the second adaptation value associated with all the cylinders to form a cylinder-selective total ignition angle in accordance with the formula EQU ZW(z)=GZ(z)-KNK(z)-AD1(z)-AD2 PA1 where z is a number of the cylinder, ZW is the total ignition angle, GZ is the basic ignition angle, KNK is the knock adjustment angle, AD1 is the first adaptation value, and AD2 is the second adaptation value. PA1 is modified in the retarded direction by a predetermined decrement after each engine cycle if the average value of all the cylinder-selective adaptation values of the current operating point has a negative sign and is greater in terms of its absolute value than the threshold; and PA1 is modified in the advanced direction by a predetermined increment after each engine cycle if the average value is less in terms of its absolute value than the threshold.